Method of calibrating a graphic arts camera

ABSTRACT

A graphic arts camera is calibrated by first determining the effective focal length of the lens employed therein, and by thereafter positioning the lens board and copy board at approximate positions corresponding to the lens conjugate of 1:1. A target, comprising a number of concentric circles the diameters of which are empirically determined to represent off-axis locations at which the effects of lateral chromatic aberration are minimal for lenses of various different effective focal lengths, is placed on the copy board, a precision ruler is superimposed on the target, and a matching precision ruler is mounted on the ground glass at the rear of the camera. The camera lens is set to wide open aperture, whereafter the copy board is adjusted, while viewing the overall target image, to focus the camera; the lens board is adjusted, using the precision rulers, to adjust the image size; and these adjustments are repeated alternatively until proper focus at a 1:1 magnification ratio is approximately achieved. The lens board is then further adjusted while observing the magnified image of a particular portion of the target, to achieve a precise focus based upon color fringing effects in that magnified image. When optimum focus at wide-open aperture and at the conjugate of 1:1 have been achieved, and checked at maximum enlargement and reduction, the pointers on the camera are set to match the scaling system or, in the case of a screw drive camera, the counters are set to the exact numbers listed in the calibration charts ordinarily associated with such cameras.

I [22] Filed:

United States Patent 1 Bender METHOD OF CALIBRATING A GRAPHIC ARTSCAMERA [75] Inventor: Lee C. Bender, Stone Park, Ill. [73] Assignee: LogEtronics, In'c., Springfield, Va.

Jan. 29', 173

[21] Appl. No.: 327,356

Primary Examiner samuel S. Matthews Assistant Examiner-Richard A.Wintercorn Attorney-Elliott I. Pollock [57] ABSTRACT A graphic artscamera is calibrated by first determining the effective focal length ofthe lens employed therein, and by thereafter positioning the lens boardJan.'l5, 1974 and copy board at approximate positions corresponding tothe lens conjugate of 1:1. A target, comprising a number of concentriccircles the diameters of which are empirically determined to representoff-axis locations at which the effects of lateral chromatic aberrationare minimal for lenses of various different effective focal lengths, isplaced on the copy board, a precision ruler is superimposed on thetarget, and a matching precision ruler is mounted on the ground glass atthe rear of the camera. The camera lens is set to wide open aperture,whereafter the copy board is adjusted, while .viewing the overall targetimage, to focus the camera; the lens board is adjusted, using theprecision rulers, to adjust the image size; and these adjustments arerepeated alternatively until proper focus at a 1:1 magnification ratiois approximately achieved. The lens board is then further adjusted whileobserving the magnified image of a particular portion of the target, toachieve a precise focus based upon color fringing effects in thatmagnified image. When optimum focus at wide-open aperture and 'at theconjugate of 1:1

I have been achieved, and checked at maximum enlargement and reduction,the pointers on the camera sociated with such cameras.

6 Claims, 2 Drawing Figures PATENTEBJAN I 5 I974 F/G. I

SHEET 1 0F 2 PATENTEBJAN 1 5 m4 sum 2 ar 2 wwo . 1 1 METHOD OFCALIBRATING A GRAPHIC ARTS CAMERA BACKGROUND orlrmz INVENTION a numberof different focal .length lenses may be,

mounted, disposed between the film plane and copy board. The camerafurther includes controls which may be employed to accurately positionthe lens board and copy board relative to one another and relative tothe film plane, in order to achieve a properly focused image of materialto be copied, at any desired ratio of magnification or reduction.Standard known formulas, which employ as parameters the focal length ofthe lens being utilized and the ratio of magnification or reductiondesired,'are used to develop data tables which may later be employed bya user of the equipment to set the lens board andcopy board positions inorder to achieve a properly focused image at a desired magnification orreduction ratio.

In order to develop the data tables, and to assure that the cameracontrols were properly calibrated in accordance with those tables, itfirst became necessary to determine the effective focal length of p thesymmetrical lens which the user mightemploy; the proper settings for thecopy board and lens board depend entirely upon that effective focallength. In point of fact, graphic arts camera manufacturers have longrecognized that the focal length specified by lens manufacturers isnominal and not exact; and it was customary to shoot each lens todetermine its effective'focal length. This was a relatively timeconsuming operation, e.g., it might require as much as four to six hoursin'order to determine the precise focal length of a givenlens, andrepresented anoperation which might have to be repeated periodically dueto physi al hanges which might occur in the camera over a long period ofusage. In addition, while techniques for shooting a lens wererelatively'accurate, they determined the average focal length for thelens. only at a particular combination of enlargement and reductionanddid not necesarily give true size and focus at the common lens conjugateof 1:1.

The first step employed in the prior shooting method was todetermine themaximum enlargement and maximum reduction that could be obtained on thecamera with the copy board in a particular fixed position. For example,with the copy board set in a given place for both enlargement andreduction, it was possible to obtain magnification ratios of 20 percentand 500 percent, or of 33% percent and 300 percent, or of percent and200 percent. The larger the spread between the enlargement andreduction, the more accurate the determination of focal length, i.e., ifthe lens in question was supposed to perform on the camera betweenmagnification ratios of 600 percent and 10 per-J cent, the copy boardwould be set at a position at which the 20 percent and 500 percentcombination of reduction and magnification could be obtained, ratherthan, for example, at the 50 percent and 200 percent combination.

Once a given enlargement and reduction combination was selected,"theperson calibrating thecam'era then place'd a focussing target on thecopy board. Prior targets employed for this purpose had a varietyofdifferent, usually arbitrary, configurations, for use in eyeballingproper focus. One such prior target configuration included a pluralityof concentric circles, ostensibly for use in focussing lenses ofdifferent focal lengths. The diameter of each circle in these priortargets was dimensioned, for example, at two-thirds of the diameter ofthe published diagonal coverage for lenses of particular focal length ata 1:1 reproduction ratio; these prior target circle diameters wererecommended by the lens manufacturers, but were pooly suited to propercalibration of the camera, and have been revised in major respects inthe target used in the present invention.

The prior target, whatever its configuration, was centered with respectto the'optical center of the copyboard, and'an accurate metal ruler(graduated in hundredths of an inch) was superimposed on the target andoriented horizontally to intersect the optical center. The operator thenroughed-in the selected enlargement at the center of the ground glass inhis camera by repositioning the lens board until the image of the targetruler appeared to be in focus and at approximately the desired size, andthen taped a matching metal ruler to the inner side of the ground glassin a position parallel to the ruler on the copy board. Once this wasdone, the

operator adjusted a magnifying loupe so that it was focused on theroughened or inner surface of the ground glass when viewing through theground glass from the clear side thereof, and thereafter commenced toadjust the copy board and lens board to produce a sharply focused imageof a designated circle of the focusing target on the ground glass of thecamera, and at the desired enlargement or reduction.

When proper focus at the selected combination of enlargement andreduction ratios was achieved, in the operators judgment, the countersettings of the lens board and copy board were noted. The effectivefocal length of the lens was then determined mathematically. Forexample, when the enlargement and reduction combination selected was 200percent and 50 percent, the mathematical technique required that theoperator first subtract the lens counter setting at 50 percent from thelens counter setting at 200 percent whereafter the remainder was dividedby 1.5, the quotient being the focal length of the lens. If thecombination of magnification and reduction selected was 500 percent and20 percent, the remainder was divided by 4.8; and if the combinationselected was 33% percent and 300 percent, the remainder was divided by2.67. The number thus computed was thereafter checked for accuracy by afurther mathematical technique.

Once the effective focal length for a given lens was computed, thatcomputed focal length was used by the camera manufacturer to preparetapes or charts which provided information or data as to just where thecopy board and lens board should be positioned by a user for thatspecific lens, to obtain a particular magnification or reduction ratioat a presumably accurate focus. The user of the equipment thereafterwould refer to these tapes or charts in setting up his camera. Inpractice,

however, it was found that, when the user resorted to the tapes orcharts alone, there would be instances where the size and/or focus werenot precise, and the user therefore was often required to make furtherminor readjustments in the lens board and/or copy board positions tocorrect the size and/or focus to his personal satisfaction. In short,the techniques employed in the past were not only time consuming, but

often resulted in calibration data which was not precisely accurate.

- The present invention, recognizing these problems of the prior art, isconcerned with a method of calibrating a graphic arts camera whichdispenses with the shooting technique discussed above, which isaccordingly far more rapid, and which can be used to calibrate a graphicarts camera to such exactitude that a user can rely with certainty uponthe calibration tables and tapes or counters associated with that cameraand can thereby dispense with the minor readjustments in copy boardand/or lens board positions which had been customary heretofore.

SUMMARY OF THE INVENTION The present invention differs from the priorshooting" technique described above, in a number of respects. First, theeffective focal length for a lens is determined by the application ofdata supplied by the lens manufacturer, as will be discussedhereinafter, to eliminate the type of mathematical computation of exactfocal length'used theretofore. In addition, proper size and focus aredetermined at the 1:1 lens conjugate, and at wide-open aperture, neitherof which conditions was characteristic of prior calibration techniques.The calibration of the equipment at the. l :l conjugate and wideopenlens aperture represents a most critical condition, and the mostdifficult image size and lens aperture setting which can be used, sincethe depth of focus is least and is most sensitive to misadjustment underthese conditions. Y

In addition, in order to achieve precise focus, the method of thepresent invention makes use of a novel target having-a number ofconcentric rings, for use respectively in focusing lenses of differenteffective focal lengths, which rings are placed at target locationsquite different from those employed in prior targets. More particularly,the target, and technique, of the present invention are intended for usewith apochromatic lenses, the type of lens most commonly employed ingraphic arts cameras at the present time; and the diameter of a givenoff-axis target ring, determined empirically for. use in focusing a lensof given nominal focal length and optical design, representsthe'off-axis location corresponding to the center of the visiblespectrum of image coloration resulting from residual lateral chromaticaberration at the Isl conjugate for any lens of that optical design andcorresponding effective focal length. v

The camera is calibrated by first determining the effective focal lengthof the lens employed therein, using manufacturer-supplieddataythereafter positioning the lens board and copy board at approximatepositions corresponding to the 1:1 lens conjugate; placing theaforementioned target onthe copy board, centered on the optical axis ofthe camera; superimposing a precision ruler on the target; mounting amatching precision ruler on the ground glass at the rear of the camera;and setting the camera lens to wide open aperture. The copy board isthen adjusted in position, while viewing the overall target image, tofocusthe camera; the lens board is adjusted in position, using theprecision rulers, to adjust image size, and these positional adjustmentsare repeated successively and alternatively until, in the operator'sjudgment, good overall focus at a 1:1 magnification ratio has beenachieved. The operator then further adjusts (if necessary) the lensboard position, while observing the magnified image of a particulartarget ring, to achieve a precise focus determined by observable colorfringing effects in the high contrast, magnified image of that targetring. When such precise focus has been achieved, and thereafter checkedat maximum enlargement and reduction ratios, the pointers on the cameraare set to match the scaling system or, in the case of a screw drivecamera, the counters are set to the exact numbers listed in theassociated calibration charts.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of agraphic arts camera set up for calibration in accordance with the methodof the present invention; and

FIG. 2 depicts, at reduced size, atarget which can be employed inpracticing the calibration method of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the subsequent description,certain lens data will be given relating to nominal focal length,corrected focal length, target ring diameters, etc. This data isapplicable to Nikkor Apochromatic Symmetric Process lenses marketed byNikon, i.e., Nippon Kogaku K. K., a Japanese company. The data, andtarget ring dimensions, must be modified when apochromatic lenses,obtained from a different source, are to be employed; and techniqueswill be discussed hereinafter to permit the derivation, for example, ofpossible target ring dimensions for use in focusing such other lenses.

Lenses of the above type are conventionally employed in graphic artscameras taking the form (subject to many variations known to thoseskilled in the art) generally shown in FIG. 1. The camera may, moreparticularly, comprise a ground glass surface 10 provided with a clear(unground) X-configuration 10a for use in viewing images of materialplaced on copy board 19. Glass surface 10 simulates the film plane inthe camera, and is pivotally mounted at 11 to permit its replacement byphotosensitive material, e. g. carried by a pivotally mounted door 12.These particular portions of the camera are normally located, along withmanually operable controls 13 and 14 used for purposes of positioningthe lens board 17 and copy board 19 (to be described) within a darkroomof light tight area represented by the wall 15; and the remainder of thegraphic arts camera projects forwardly of said wall 15.

The camera further includes guideways such as 16 upon which a lens board17, associated with a bellows 18, may be mounted for movement under thecontrol of an operator; and a copy board 19 is also mounted on saidguideways, forwardly of the lens board 17, for similar movement underoperator control. The copy board 19 is normally associated withappropriate lamps such as 20 for reflectively illuminating the copyboard.

The lens board 17 (upon which any of various different focal lengthlenses may be mounted) and the copy board 19 are adapted to be preciselypositioned by controls l3 and 14 in order to achieve proper focus at adesired magnification (reduction or enlargement) ratio. Moreparticularly, the boards 17 and 19 may be adjustably positioned byprecision screw drives which are associated with counters disposedadjacent the controls lated until specified counts are achieved (inaccor-' dance with pre-prepared calibration tables) to ostensiblyachieve proper positioning of the lens board 17 and copy board 19 for adesired application. Alternatively, the controls 13 and 14 can beprovided with or associated with pointers which are, in turn, adapted tocooperate with an appropriate scaling system, such as are made up andused. When the camera scaling system I using focusing tapes, these areinstalled in the process precision graduated tapes coupled respectivelyto the lens board 17and copy board 19 andmovable therewith past saidpointers as the controls 13 and 14 are manipulated. These arrangements(not shown in the drawings) and other arrangements known to thoseskilled in the art for these purposes, will be termed hereinafter thecamera scaling system.

The actual position which the lens board 17 and copy board 19 shouldtake relative to one another, and relative to the film plane defined bythe forward or rough: ened surface of ground glass 10, to achieve aproperly focused'image of specified enlargement or reduction size, canbe computed by standard formulas; and such formulas are customarily usedto generate data tables which are ther'eafterused by the camera operatorto achieve a desired setup of the camera. Before the operator can trulyrely upon the tables, however, the camera must be calibratedi.e.,-th'ecamera scaling system must be so set that an operator, by manipulatingcontrols l3 and 14 until a particular count or scaleposition isachieved, will thereby correctly locate the copy board 19 and lens board17 attheprecise positions relative to one another and relative to thefilm plane necessary to achieve the desired results. The presentinvention, as discussed earlier, is intended to permit a cameramanufacturer or user to achieve-such calibration with far greaterprecision and accuracythan has been the case heretofore.

The first step in the calibration method of the present invention is todetermine the effective focal'lengthof the lens which is mounted on lensboard 17. This step of the method is effected by use of data supplied bythe lens manufacturer (in this case Nikon).'Every lens supplied by themanufacturer is accompanied by a lens data card which lists manycharacteristics of that particular lens; but the only data which needbeconsidered at this point is the nominalfocal-length. The nominal focallength, listed in inches, minus a so-calle'd Delta f1 factor in inches,will provide the efi'ective focal length of that lens. The Deltaflfactor for a representative group of apo-chromatic Nikkor Symmetriclenses is as follows:

Nominal Focal Deltafl Factor Regardless of the enlargement and reductionapplication of the lens in question, theDelta fl factor will compensatefor the entire range .of focus change, provided that the lens wasproperly sized and focused at the 1:1 conjugate.

,After the effective focal length of the lens has been determined by useof the above data, charts and tapes camera. The camera is squared toobtain planar parallel relationships between the film, lens board, andcopy board, to within $0.002 inches, and the ground glass is squared andits front surface set to +0.005 inches from the film supporting surfaceto allow for film thickness.

The lens is then mounted on the lens board 17, and the lens board is setto approximately two times the effective focal length (previouslydetermined) from the film plane (for example, if we assume that theeffective focal length is 24 inches, the lens would be positioned 48inches from the film plane). The copy board is in turn set toapproximately four times the lens focal length from the film plane (forexample, using the 24 inch focal length lens assumed above, the copyboard would be set 96 inches from the film plane). By these initialsettings, the lens board and copy board are set close to the 1:1conjugate, i.e., any material placed on copy board 19 will be imaged onthe front surface of ground glass 10 at approximately the same size.

A high-contrast (e.g., black lines on white background) target 21 (shownin detail but at reduced scale in FIG. 2), is then placed on copy board19. Target 21 comprises a plurality of concentric rings A, B, C, etc.,having different diameters, and each ring bears a designation thereonidentifying a nominal lens focal length associated therewith, e. g.,ring A is identified by the numeral l2 and is to be used, aswill bedescribed hereinafter, for precision focusing a lens having a nominalfocal length of 12 inches. Similarly, ring B is used in conjunction witha 14-inch focal length lens; ring C is used with a 19-inch focal lengthlens, etc. The target 21 further includes a checkerboard pattern D of X-configuration which is centered on the various target rings. Thecheckerboard pattern simulates a half-tone representation, is shaped tocoincide with the clear glass portion 10a of the camera ground glass 10,and

. orientations of the various planes specified until the same degree ofresolution is obtained at all four corners of the image of target 21.The NBS resolution targets can be omitted, however, and indeed thecheckerboard pattern D can also be omitted, since they have no directrelationship to the precision focusing technique accomplished throughthe use of the rings A, B, C, etc.

The diameter of each target ring A, B, C, etc. is determined empiricallyby a technique which will be described hereinafter, and represents theoff-axis locus of points at which the effects of lateral chromaticaberration are minimum for the lens designated by that ring. The radiusof each ring will be dependent upon the lens characteristics and will,accordingly, be different for lensesm'anufactured by differentcompanies. The following chart of given nominal lens sizes, and targetring radii, is applicable to the aforementioned Nikkor lenses:

Nominal Lens Target Ring Radius After the lens board and copy board ofthe camera have been set to the 1:1 conjugate position as describedpreviously, and target 21 has been placed on the copy board and centeredwith respect to the optical center of the lens, a precision ruler 22,graduated in increments of 0.010'inches (e.g., a ruler of the typemanufactured by LS. Starrett Company, Athol, Mass), is placed upon thetarget 22 and attached thereto adjacent the optical center of the targetin accurate horizontal orientation. A matching precision ruler 23 isthen mounted and taped to the inner or roughened side ofthe ground glass10 in a position such that it is accurately aligned with the image 22aof the precision ruler 22. Each of the rulers 22 and 23 can be, forexample, 24-inch rulers. The lens on lens board 17 is now set to wideopen aperture, and reflective lighting, e.g., using lamps 20, isprovided to achieve'a maximum contrast between the black and white areasof the high-contrast target 21. The lighting is set by moving the lights20 until a uniformly illuminated high contrast image appears on' theground glass 10.

The foregoing steps of the procedure, which could be termed the initialsetup steps, are now followed by a further series of steps which aretermed the rough sizing and focusing steps. More particularly, theoverall target image is now viewed on ground glass 10 and, if the imageappears to be out of focus, the image is brought into rough focus bymoving copy board 19 toward or away from the film plane defined byground glass 10. The checkerboard portion D of the target, as viewed inthe clear glass portion 10a of ground glass 10, is particularly usefulin achieving this rough focus.

' When the rough focus has been achieved, in the judgment of theoperator, rough size must again be set, i.e., the target image must havethe same size as the target itself. In order to set rough size, thecenter graduation mark (e.g., the 12-inch mark) of ruler 23 isaccurately aligned with the like mark in the projected image 22a oftarget-mounted ruler 22 and, after the marks have been so centered, thelens board 17 is moved toward or away from copy board 19 until theextreme right and left edge ruler marks (e.g., the l-inch and 23-inchgraduations) in ruler 23 and image 22a are also accurately aligned withone another. When the 1:1 magnification ratio has thus been achieved,the image will probably be slightly out, of focus; and the focus isaccordingly again set by the procedure previously described, i.e., bymoving copy board 21. This focus readjustment may, in turn, require thatthe size be reset in accordance with the procedure discussed; and thesealternate adjustments in the copy board and lens board position arecontinued until a proper size within 10.0015 inches, at good focus ofthe overall target image, is achieved.

The operator then proceeds to a further series of steps to obtainprecise focus and size. This series of steps requires that the operatorconcentrate on the particular one of the target rings A, B, C, etc.,which corresponds to the nominal focal length of the lens then mountedon lens board 17. The image of that particular ring is viewed throughthe clear glass portion 10a of ground glass 10, under magnification,e.g., by supporting on the rear surface of ground glass 10 a 10Xmagnifier or loupe 24 which is focused on the roughened front surface(or image plane) of the ground glass 10. When the target ring image,thus viewed under magnification, is at exactly a 1:1 magnification ratioi 0.0015 inches, and in precise focus, the image will appear to have afringe of brownish tint, rather than of red or green tint. Statedanother way, precise focus is determined by noting certain colorfringing effects which occur in the image of the particular target ringunder observation, and precise focus corresponds to a condition whereinthe color being observed is substantially at the center of the visiblespectrum. if this brownish fringe tint is not observed, i.e., if thecolor of the target ring image fringe appears to be red or green, thelens board 17 and copy board 19 must be readjusted sequentially andalternatively, by the rough sizing and focusing steps previouslydescribed, until the desired precise focus is achieved at the 1:1conjugate.

To further check the calibration of the lens, the lens board 17 and copyboard 19 should now be set to relative positions corresponding tomaximum enlargement and maximum reduction ratios, and the magnifiedtarget ring image should again be viewed on ground glass 10 for propersize and focus. If the focus is unsharp at maximum enlargement ormaximum reduction, the operator must he go back to the 1:1 setting andrepeat the entire procedure. Similarly, if the size varies by more than0.0035 inches at maximum enlargement and maximum reduction, he shouldalso go back to the 1:1 setting and repeat the entire procedure.

When precise focus at the 1:1 conjugate has been achieved, and checkedin the foregoing manner, the camera scaling system is set and lockedinto proper position. More particularly, once exact focus has beenobtained at the 1:1 magnification ratio, the operator consults thepreviously-prepared, mathematically computed, calibration tablesassociated with the camera to determine where, according to thosetables, the lens board and copy board should be" positioned; and thecamera scaling system is then adjusted (e.g., the pointers, if used, orcounters, if used, are set) to the precise readings specified in thecalibration table, and then secured, e.g., pinned, at those precisesettings.

The target 21, and more particularly the radii of the various targetrings therein, is developed empirically by a procedure entirely similarto that described above. A group of lenses (e.g., 30 lenses) of eachnominal focal length is obtained from a manufacturing source for each ofthe focal lengths of interest (i.e., if six nominal focal lengths areunder consideration, a total of something in the order of lenses shouldbe used). The Delta fl factor for each nominal focal length is obtainedfrom the lens manufacturer, and the effective focal length of the lensesin each group is calculated. Then, working with all of the lenses in agroup of a particular nominal focal length, each such lens is mounted onthe lens board, the lens is opened to its maximum aperture, acheckerboard pattern of the type represented at D in FIG. 2 is mountedon the copy board, precision rulers are mounted as previously described,and the lens board and copy board are adjusted in position relative toone another to obtain 'an accurately focused and sized image on theground glass at a precise 1:1 magnification ratio.

When each lens is positioned at the 1:1 conjugate, a color spectrum willbe observed in the image area appearing in the clear glass portion a ofground glass 10. The spectrum will progress from a red tint near theoptical center of the image area to a green tint radially outward ofthat optical center. Points are then marked in each of the fourquadrants (represented by the legs of the X-portion 10a) of the groundglass 10, with each marked point corresponding to the approximatemidpoint of the visible spectrum (i.e., the brownish"tint referred toearlier) observed in agiven quadrant. The lens then mounted in the lensboard 17 is replaced by a different lens of the same nominal focallength, the same procedure is followed, mid-spectrum points are markedin the four quadrants for this substituted lens, etc. After each of thee.g., 30 lenses in the group has been checked in this fashion, thespread of points in each quadrant, relative to the optical center isnoted, an average point location is determined in each of the fourquadrants, and a line is scribed through those se lected average pointsto produce a target ring (such as A) for later use with lenses of thatfocal length and source. By an entirely similar empirical procedure, thetarget rings B, C, etc., can be located and scribed onto the target.

It must be noted that the location of the various target rings producedby the foregoing procedure is quite different from that of priortargets. For example, the diagonal'coverage of a Nikkor apochromaticlens having a nominal focal length of 12 inches, at its maximum "aerture (f/9) and at a l l reproduction ratio, is

inches. In prior targets used for calibration purposes, it was assumedthat the diameter of the target ring to be used in precise focusing ofthe lens should be two-thirds the diagonal coverage, i.e., for theparameters described, the target ring of the 12 inch focal length lenswould be approximately 14 inches in diameter. In contrast, the diameterof the ring A is, as described previously, only 3 inches. Similarly,using lens data for a 19 inch focal length lens, prior techniques wouldhave selected a target ring of approximately 21.5 inches in diameter,whereas the diameter of the. ring C in the target employed in thepresent invention is only six inches. In short, the various ringdiameters embodied in the target 21 are markedly different from thoseconsidered proper theretofore, are generated on a basis quite differentfrom that selected heretofore, and, when used in the overall adjustmentprocedure described, permit the camera to becalibrated to far greateraccuracy than was possible heretofore.

While I have thus described a preferred embodiment of the presentinvention, many variations will be suggested to those skilled in theart. It must therefore be understood that the foregoing description isintended tobe illustrative only and not limitative of the presentinvention. All such variations'and modifications as are in accord withthe principles described are meant to fall within the scope of theappended claims.

Having thus described my invention, 1 claim:

1. A method of calibrating a graphic arts camera, of the type comprisinga ground glass surface defining a film plane, a movable copy board, amovable lens board disposed between said ground glass surface and copyboard, and controls including a camera scaling system for positioningsaid copy board and lens board relative to one another and relative tosaid film plane; said method comprising the steps of preparing a targetwhich includes a first indicia representing an optical axis and a secondhigh contrast indicia displaced from 7 said first indicia and positionedat an off-axis location at which the effects of chromatic aberration foran apochromatic lens of particular effective focal length are minimum ata lens conjugate of 1:1; mounting an apochromatic lens of said effectivefocal length on the lens board, mounting said target on said copy boardwith the optical axes of said lens and target in alignment with oneanother and with the said copy board, lens board and ground glasssurface in plane parallelism, setting said lens to its maximum aperturesize, adjusting the positions of said lens board and copy board to the1:1 lens conjugate, viewing color fringing effects in a magnified imageof said second indicia on said ground glass while conducting saidadjusting step until the observed color in said magnified imagecorresponds substantially to the center of the visible spectrum, andthereafter setting the camera scaling system.

2. The method of claim 1 wherein said step of preparing a targetcomprises locating a plurality of different second indicia differentlydisplaced from said first indicia at a plurality of different off-axislocations at which the effects of chromatic aberration for a pluralityof apochromatic lens of different effective focal lengths respectivelyis minimum, saidviewing step comprising the step of viewing a magnifiedimage of a particular one of said second indicia associated with thefocal length of the lens then mounted on said lens board.

3. The method of claim 2 wherein said adjusting step comprises mountinga first precision ruler on said copy board to image both said firstruler and said target on said ground glass surface, mounting a secondprecision ruler on the roughened interior surface of said ground glassin parallel relation to the image of said first ruler, repositioningsaid copy board while viewing the second ruler and imaged first ruler onsaid ground glass surface to adjust image size, and repositioning saidlens board 'while viewing said imaged targeton said ground glass 5. Themethod of claim 2 wherein said step of prepar ing said target comprisesgenerating a plurality of different diameter circles, concentric withone another and with said first indicia, to produce said plurality ofsecond indicia.

6. The method of claim 5 wherein said step of preparing said targetfurther comprises the step of superimposing a high-contrast checkerboardpattern on said plurality of circles, said viewing step comprisinginitially viewing the image of said checkerboard pattern on said groundglass surface during said adjusting step and thereafter viewing themagnified image of a portion of a selected one of said concentriccircles during a continuation of said adjusting step.

1. A method of calibrating a graphic arts camera, of the type comprisinga ground glass surface defining a film plane, a movable copy board, amovable lens board disposed between said ground glass surface and copyboard, and controls including a camera scaling system for positioningsaid copy board and lens board relative to one another and relative tosaid film plane; said method comprising the steps of preparing a targetwhich includes a first indicia representing an optical axis and a secondhigh contrast indicia displaced from said first indicia and positionedat an off-axis location at which the effects of chromatic aberration foran apochromatic lens of particular effective focal length are minimum ata lens conjugate of 1:1; mounting an apochromatic lens of said effectivefocal length on the lens board, mounting said target on said copy boardwith the optical axes of said lens and target in alignment with oneanother and with the said copy board, lens board and ground glasssurface in plane parallelism, setting said lens to its maximum aperturesize, adjusting the positions of said lens board and copy board to the1:1 lens conjugate, viewing color fringing effects in a magnified imageof said second indicia on said ground glass while conducting saidadjusting step until the observed color in said magnified imagecorresponds substantially to the center of the visible spectrum, andthereafter setting the camera scaling system.
 2. The method of claim 1wherein said step of preparing a target comprises locating a pluralityof different second indicia differently displaced from said firstindicia at a plurality of different off-axis locations at which theeffects of chromatic aberration for a plurality of apochromatic lens ofdifferent effective focal lengths respectively is minimum, said viewingstep comprising the step of viewing a magnified image of a particularone of said second indicia associated with the focal length of the lensthen mounted on said lens board.
 3. The method of claim 2 wherein saidadjusting step comprises mounting a first precision ruler on said copyboard to image both said first ruler and said target on said groundglass surface, mounting a second precision ruler on the roughenedinterior surface of said ground glass in parallel relation to the imageof said first ruler, repositioning said copy board wHile viewing thesecond ruler and imaged first ruler on said ground glass surface toadjust image size, and repositioning said lens board while viewing saidimaged target on said ground glass surface to adjust focus.
 4. Themethod of claim 3 wherein said steps of repositioning said copy boardand repositioning said lens board are conducted in alternative repeatedsequence.
 5. The method of claim 2 wherein said step of preparing saidtarget comprises generating a plurality of different diameter circles,concentric with one another and with said first indicia, to produce saidplurality of second indicia.
 6. The method of claim 5 wherein said stepof preparing said target further comprises the step of superimposing ahigh-contrast checkerboard pattern on said plurality of circles, saidviewing step comprising initially viewing the image of said checkerboardpattern on said ground glass surface during said adjusting step andthereafter viewing the magnified image of a portion of a selected one ofsaid concentric circles during a continuation of said adjusting step.